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/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.rsocket.internal.jctools.queues;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.util.concurrent.atomic.AtomicReferenceArray;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import sun.misc.Unsafe;

/**
 * Why should we resort to using Unsafe?

 *
 * 
 *   To construct class fields which allow volatile/ordered/plain access: This requirement is
 *       covered by {@link AtomicReferenceFieldUpdater} and similar but their performance is
 *       arguably worse than the DIY approach (depending on JVM version) while Unsafe
 *       intrinsification is a far lesser challenge for JIT compilers.
 *   
To construct flavors of {@link AtomicReferenceArray}.
 *   
Other use cases exist but are not present in this library yet.
 * 
 *
 * @author nitsanw
 */
class UnsafeAccess {
  public static final boolean SUPPORTS_GET_AND_SET_REF;
  public static final boolean SUPPORTS_GET_AND_ADD_LONG;
  public static final Unsafe UNSAFE;

  static {
    UNSAFE = getUnsafe();
    SUPPORTS_GET_AND_SET_REF = hasGetAndSetSupport();
    SUPPORTS_GET_AND_ADD_LONG = hasGetAndAddLongSupport();
  }

  private static Unsafe getUnsafe() {
    Unsafe instance;
    try {
      final Field field = Unsafe.class.getDeclaredField("theUnsafe");
      field.setAccessible(true);
      instance = (Unsafe) field.get(null);
    } catch (Exception ignored) {
      // Some platforms, notably Android, might not have a sun.misc.Unsafe implementation with a
      // private
      // `theUnsafe` static instance. In this case we can try to call the default constructor, which
      // is sufficient
      // for Android usage.
      try {
        Constructor c = Unsafe.class.getDeclaredConstructor();
        c.setAccessible(true);
        instance = c.newInstance();
      } catch (Exception e) {
        throw new RuntimeException(e);
      }
    }
    return instance;
  }

  private static boolean hasGetAndSetSupport() {
    try {
      Unsafe.class.getMethod("getAndSetObject", Object.class, Long.TYPE, Object.class);
      return true;
    } catch (Exception ignored) {
    }
    return false;
  }

  private static boolean hasGetAndAddLongSupport() {
    try {
      Unsafe.class.getMethod("getAndAddLong", Object.class, Long.TYPE, Long.TYPE);
      return true;
    } catch (Exception ignored) {
    }
    return false;
  }

  public static long fieldOffset(Class clz, String fieldName) throws RuntimeException {
    try {
      return UNSAFE.objectFieldOffset(clz.getDeclaredField(fieldName));
    } catch (NoSuchFieldException e) {
      throw new RuntimeException(e);
    }
  }
}